FIG. 1 illustrates how existing fire fighting equipment is typically upgraded with a foam proportioning system (FPS) 18. The existing fire fighting equipment can include a programmable logic controller (PLC) 10 controlled by a library 12, which receives input by communicating with a support element 14 of a display 16. The FPS 18 can include a foam proportioner 20 and a controller 22 with a display 24. The controller 22 must be linked to the fire apparatus PLC 10 and the fire apparatus display 16 with a communication line 26. Because no modifications to the fire apparatus PLC 10 are generally performed, the fire apparatus PLC 10 may not be able to communicate with the foam proportioner's controller 22, resulting in the possibility of duplicate input signals.